FIELD OF THE INVENTION
This invention relates to an image processing apparatus which may be utilized in a facsimile apparatus, an electronic filing apparatus or the like, and more particularly, to an image processing apparatus which performs image encoding and decoding according to hierarchical encoding.
In a facsimile apparatus which is a typical conventional static image communication apparatus, a system is adopted in which images are sequentially scanned in a raster direction, encoded and transmitted. Since every pixel of the image must be encoded and transmitted in order to send the entire image, transmission takes a long time, and therefore, the application of the system to image communication services, such as an image database service, videotex or the like, is difficult.
In order to transmit an overall image quickly, hierarchical encoding has been devised by the JBIG (Joint Bi-level Image Experts Group, which is an organization of the ISO/CCITT (International Organization for Standardization/Comite Consulatif International Telegraphique et Telephonique)). An example of conventional hierarchical encoding is shown in FIG. 9. In this example, an original image to be encoded has a resolution of 400-dpi (dots per inch). In FIG. 9, image memories 701 through 706 are provided for storing 400-, 200-, 100-, 50-, 25- and 12.5-dpi images, respectively. Reduction units 719 through 723 provides 200-, 100-, 50-, 25- and 12.5-dpi images, respectively, and encoders 707 through 712 encode 400-, 200-, 100-, 50-, 25- and 12.5-dpi images, respectively.
The encoded 400-, 200-, 100-, 50-, 25- and 12.5-dpi images are stored in disk memories 713 through 718, respectively.
Reduction unit 719 reduces a 400-dpi image, from image memory 701, by a technique in which the image is subsampled to 1/2 in both the main-scanning and sub-scanning directions to provide a 200-dpi image, which is stored in image memory 702. The above mentioned subsample process is not a simple subsampling as default, but a specially programmed JBIG standard method. The 200-dpi image is further reduced by reduction unit 720 to provide a 100-dpi image, which is stored in image memory 703. Likewise, 50-, 25- and 12.5-dpi images are provided and are stored in image memories 704, 705 and 706, respectively.
The images, which are stored in disk memories 713 through 718, are transmitted, in turn, in codes beginning with lower resolution images so as to give a rough overall image. Namely, the images are transmitted in the order of 12.5-, 25-, 50-, 100-, 200- and 400-dpi images.
To encode the 12.5-dpi image, the image stored in image memory 706 is scanned and entropy encoding (such as arithmetic encoding) is performed by referring to an object pixel to be encoded and the surrounding pixels. As regards the 25-dpi image, encoding is performed by encoder 711 by referring to pixels surrounding a target pixel from image memory 705, and surrounding pixels of the 12.5-dpi image from image memory 706, so that encoding efficiency is improved.
Likewise, as regards the 50-dpi image of image memory 704, the 25-dpi image of image memory 705 is referred to and encoded by encoder 710; as regards the 100-dpi image of image memory 703, the 50-dpi image of image memory 704 is referred to and encoded by encoder 709; as regards the 200-dpi image of image memory 702, the 100-dpi image of image memory 703 is referred to and encoded by encoder 708; and, as regards the actual-size (400-dpi) image of image memory 701, the 200-dpi image of image memory 702 is referred to and encoded by encoder 707.
In a typical configuration as shown in FIG. 9, however, it is necessary to provide image memories 701 through 706, corresponding to respective resolutions, thereby causing an increase in the production cost. Furthermore, since data for several lines are used as reference pixels for image reduction and encoding for both high-resolution and low-resolution images, interfacing with the image memories becomes complicated.